IMMUNOLOGY OF TRANSPLANTATION

Lecture 7 2013/2014 Jan Żeromski

POINTS TO BE DISCUSSED

Immunogenetics – basic facts Histocompatibility antigens and their role in transplantation Types of grafts, including fetus Mechanisms of graft rejection The tempo of rejection Prevention of rejection Graft versus host reaction

MAJOR HISTOCOMPATIBILITY COMPLEX (MHC)

 Is located on short arm of chromosome 6  It includes 3 regions

: class Ia

(loci A, B, C)

class Ib

(loci E, F, G, H), DP) and

class III class II

(loci DR, DQ,  Genes of class Ia and class II are highly polymorphic, while those of class Ib and class III are not  Polymorphism means occurence of several allelles ie.genes encoding various MHC antigens located at the same locus

MAJOR HISTOCOMPATIBILITY ANTIGENS

Histocompatibility antigens are cell surface expressed on all cells (class I) and on APC, B cells, monocytes/macrophages (class II) They are targets for rejection They are inherited from both parents as MHC haplotypes and are co-dominantly expressed

MINOR HISTOCOMPATIBILITY ANTIGENS

They also participate in rejection but to lesser degree Disparity of several minor antigens may result in rejection, even when MHC antigens are concordant between donor and recipient They include blood group antigens, tissue and organ antigens, normal cellular constituents They are peptides derived from polymorphic cellular proteins bound to MHC class I molecules

TYPES OF GRAFTS

Autologous graft (autograft)

– in the same individual: from one site to another one

Isogenic (isograft)

– between genetically identical individuals

Allogeneic (allograft or homograft)

– between different members of the same species

Xenogeneic (xenograft)

– between mmbers of different species

MECHANISMS OF REJECTION

Depend on disparity of genetic background between donor and recipient T cells are critical in graft rejection Rejection responses in molecular terms, are due to TCR-MHC interaction Graft and host MHC molecules present different peptides Different MHC molecules have different peptide binding grooves T lymphocytes can directly recognize and respond to foreign MHC molecules

ALLOREACTIVE CELLS ARE SO COMMON, BECAUSE:

Foreign MHC molecules differ from self MHC at multiple different aminoacid residues, each of which may produce determinant recognized by a different cross-reactive T cell clone Thus, each foreign MHC molecule is recognized by multiple clones of T cells 2% of host T cells are capable recognizing and responding to a single MHC foreign molecule

TEMPO OF REJECTION

Hyperacute rejection

antibodies to HLA and ABO blood group system (hours or first days)

Acute rejection

T cells (days or weeks)

Chronic rejection

various mechanisms: cell-mediated, deposition of antibodies or antigen antibody complexes with subsequent obliteration of blood vessels and interstitial fibrosis (months or years)

THE MODE OF ACTION OF ANTIBODIES IN TRANSPLANT REJECTION

By damage of endothelial cells due to activation of complement By induction of ADCC reaction Through intensification of inflammatory reaction by the release of complement components (C3a, C5a) By activation of clotting system

VARIABLES DETERMINING TRANSPLANT OUTCOME

Donor-host antigenic disparity Strength of host anti donor response Immunosuppressive regimen The condition of the allograft Primary disease of the host

PATHOGENESIS OF CHRONIC REJECTION

Is the result of organ damage by immunologic and non-immunologic factors Initially – the minor damage and activation of endothelium by cytotoxic T cells and antibodies

PATHOGENESIS OF CHRONIC REJECTION -2

Production by endothelial cells biologically active mediators (PDGF, PAF, TNF, thromboxans etc.) Secretion of cytokines by infiltrating lymphocytes Mitogenic effect on myocytes and fibroblasts results in cell proliferation and fibrosis

CHRONIC REJECTION IS MORE FREQUENT WHEN:

Were previous episodes of acute rejection There is a low number of compatible HLA antigens with recipient Patient on inadequate immunosuppression Recipient is hypertonic >>>

CHRONIC REJECTION IS MORE FREQUENT WHEN:

<<< In the case of cytomegaly virus infection The period of organ storage was too long Patient is heavy smoker and/or is hyperlipidemic Organ mass is unproportionally small as compared to body mass

MODERN IMMUNOSUPPRESSIVE THERAPY Cyclosporin

(CsA),

Tacrolimus

(FK-506) – inhibit IL-2 production by T cells calcineurin antagonist

Sirolimus (rapamycin

) – inhibits signals transmitted by IL-2 binding to IL-2R (antiproliferating effect)

Azathioprine

– reduces numbers and function both, T and B cells, by inhibition of purine metabolism

MODERN IMMUNOSUPPRESSIVE THERAPY -2 Mycophenolate mofetil

(MMF) – inhibits DNA synthesis and protein glycosylation, supresses expression of CD25, -71, -154, -28.

Anti-IL-

2 monoclonal antibodies

FTY 720

migration – dramatic effect on lymphocyte

GRAFT VERSUS HOST DISEASE (GVH)

Is common complication in recipients of bone marrow transplants Is due to the presence of alloreactive T cells in the graft It results in severe tissue damage, particularly to the skin and intestine

GRAFT VERSUS HOST DISEASE (GVH)

It may be avoided by careful typing, removal of mature T cells from the graft and by immunosuppressive drugs It is manifested by marked rise of several cytokines in patient’s serum (IFN  , TNF, IL-1, IL-2, IL-4)

RISK FACTORS IN FORMATION OF GVH

Acute GVH

Previous pregnancies in female donor High T cell number in marrow HLA disparity Transplant from female to male Low immunosuppression Herpes virus infection

Chronic GVH

Aging of donor and recipient Donor’s leukocyte transfusion Previous acute GVH High dosage radiation Transplant from female to man HLA disparity

GRAFT VERSUS LEUKEMIA (GVL)

It is reaction of donor’s lymphocytes from bone marrow graft versus antigens on tumor cells It is postulated that apart from GVH, there exists independent GVL component GVL is due to T cells and NK cells Relapses of leukemia are rarer in those patients, who experienced GVH after marrow transplant GVL may be enhanced by transfusion of lymphocytes obtained from marrow donors +IL-2

The fetus is allograft that is tolerated repeatedly

Fetus carries paternal MHC and minor H antigens that differ from those of mother Still, fetus is an allograft that is not rejected Women who born several children make antibodies directed at father’s MHC proteins Possible explanations of this puzzle:    Lack of classic MHC antigens on cells of trophoblast cells (TC) (protection from maternal T cells) Presence of HLA-G antigens on TC (protection from NK cells) Secretion of suppresive cytokines by TC and uterine epithelium (TGF-beta, IL-10, IL-4)

PERSPECTIVES OF XENOGENEIC GRAFTS

Potential advantage due to larger accessibility of animal organs Monkeys are apparently the most suitable donors, but dangerous because of potential risk of retrovirus transfer within graft

PERSPECTIVES OF XENOGENEIC GRAFTS - 2

Pigs are now considered because of similar sizes of organs and erythrocytes to human ones The major obstacle – presence in man (1%) of natural antibodies vs. Gal (galactose  -1,3-galactose) causing hyperacute rejection

Thank you for your attention and perseverance

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